Subpopulations of lymphocytes may be defined on the basis of functional, morphological and immunochemical differences. Included among the properties that distinguish thymus-derived (T) from bone marrow-derived (B) lymphocytes are specific receptor sites, homing patterns in lymphoid tissues, traffic in the intact host, the topographical appearance of the cell surface and responses to specific mitogens. These differences between T and B cells presumably reflect discrepancies in the chemical composition and/or structural arrangement of the plasma membrane. B cells may be further divided into subpopulations on the basis of membrane immunoglobulins. More recently, subpopulations of T cells have also been defined on a functional basis. Investigations to date suggest that most, if not all, subpopulations of lymphocytes differ in radiosensitivity. Thus: (1) irradiated B cells placed in tissue culture survive less well than T cells; (2) the capacity of irradiated cells to traffic in normal fashion is inhibited at lower dose levels for B than T cells; (3) after whole body exposure to 800 rads, a proportionately greater number of T than B cells are mobilized via thoracic duct cannulation; (4) T and B cells activated to appropriate mitogens or antigens in vivo or in vitro are more resistant than their non-activated counterparts to radiation-induced interphase cell death; (5) suppressor T cells appear to be more radiosensitive than helper T cells; (6) B cells demonstrate radiation-induced morphologic alterations at lower dose levels than do T cells; (7) cortisone-sensitive T and B lymphoid cell lines are more radiosensitive than cortisone-resistant cell lines; (8) a subpopulation of cytotoxic T cells is very resistant to radiation injury. The purpose of this project is twofold: expand the above observations on the radiosensitivity of defined populations of lymphocytes; define the physical-chemical basis of these differences in radiosensitivity and thereby gain a better understanding of the pathogenesis of radiation-induced interphase death of lymphocytes.